1. Field of the Invention
The present invention relates to an islanding detection apparatus for a distributed generation power system and a detection method therefor. Particularly, the present invention relates to the islanding detection apparatus employing a power converter to connect with a utility power system for detecting power failure of the utility power system. More particularly, the present invention relates to the islanding detection apparatus used to disconnect the distributed power system from the utility power system for avoiding islanding operation.
2. Description of the Related Art
Generally, small capacity distributed generation power systems such as small capacity power stations or renewable power stations serve in some energy markets. Currently, renewable energies suitable for generating electric power include solar power, wind power and fuel cell power etc. In the future various types of small capacity power stations will be incorporated into an ordinary distributed generation power system.
The small capacity distributed generation power systems are directly incorporated into a utility power system for supplying electric power to loads. Conventionally, there are some protective methods for the distributed power system, including detection for electric power quality and islanding operation of a distributed generation power system. A phenomenon so-called “islanding operation” is that a utility power system may be cut off due to the power failure or maintenance of electrical equipments, but a small capacity distributed generation power system can still supply electric power as usual. Accordingly, this may cause distributed generation power systems to be supplied electric power individually. Many problems caused by the phenomenon of islanding operation are given as follows:                1. Islanding operation may jeopardize public security or endanger maintenance workers working in the area of the distributed generation power system;        2. Islanding operation may cause unregulated voltage and frequency of electric power of the distributed generation power system so that electrical components and equipments located in the area of the distributed generation power system may be damaged;        3. Islanding operation may cause malfunction of protection relays located in the area of the distributed generation power system; and        4. Once the utility power system is re-supplying electric power, islanding operation may cause asynchronous problem of electric power between the distributed generation power system and the utility power system.        
Hence, many islanding operation control standards, such as IEEE929, IEEE1547, VDE0126 and UL1741 etc., have been established in Europe, United State of America, Japan and other international countries.
The detection methods for islanding operation can be divided into an active detection method and a passive detection method. The passive detection methods are used to detect changes in parameters of a power system for determining whether the islanding operation occurs. For example, the passive detection methods include a system-frequency detection method, a voltage-amplitude detection method, a harmonic-contained detection method and so on. However, there is a dead-zone existing in these passive detection methods. The term “dead-zone” means a situation that these passive detection methods cannot detect islanding operation occurring in the power system. Accordingly, these passive detection methods cannot meet the requirements of the islanding control standards.
As to the active detection method, a small fluctuation is added in an output current of the distributed generation power system. When the utility power system is normal, the small fluctuation cannot affect the distributed generation power system due to the strong utility power system. Conversely, when the utility power source is interrupted, the small fluctuation can cause a great change in frequency or voltage. In this way, a protection relay can immediately detect such a change and judge it as islanding operation. Instantly, the distributed generation power system must be disconnected from the utility power system so as to avoid occurrence of islanding operation.
A conventional active detection method described in U.S. Pat. No. 5,493,485 discloses the islanding detection for the distributed generation power system by means of its phase shift and harmonic distortion. Accordingly, an occurrence of the islanding operation in the distributed generation power system can be detected. Another conventional active detection method described in U.S. Pat. No. 6,172,889 discloses the islanding detection for the distributed generation power system by generating a frequency shift in an output current of the distributed generation power system. Accordingly, an occurrence of the islanding operation in the distributed generation power system can be detected by a positive feedback manner. Another conventional active detection method described in Taiwanese Patent Publication No. 430,731 discloses the islanding detection for the distributed generation power system by generating an asymmetric output current of the distributed generation power system. Accordingly, an occurrence of the islanding operation in the distributed generation power system can be detected. Another conventional active detection method described in U.S. Pat. No. 6,603,290 discloses the islanding detection by generating an amount of harmonics in the output current of the distributed generation power system. Accordingly, an occurrence of the islanding operation in the distributed generation power system can be detected by measuring impedance of the distributed generation power system.
As have been discussed above, these active detection methods employ a small distortion in the output current of the distributed generation power system. In this way, the islanding operation in the distributed generation power system due to power interruption of the utility power system can be detected. Nevertheless, these active detection methods must comply with all international islanding control standards such that total harmonic distortion of a current supplied from the distributed generation power system must be less than 5%. Hence, the harmonic distortion resulted from these active detection methods must be restricted by the islanding control standards so that detection time of islanding detection is increased and operating reliability is affected. However, there is a dead-zone also existing in these active detection methods. Furthermore, a control method employed in these active detection methods may be sophisticated.
The present invention intends to provide an islanding detection apparatus and a detection method to act a distributed power system as virtual impedance under abnormal condition of the utility power system. The virtual impedance is operated at a frequency close to but unequal to that of a utility power system. When the utility power system is normal, a voltage of the utility power system is identical with that of a load. In this manner, the distributed generation power system generates a real power, and its output current is a sine-wave signal whose frequency and phase are identical with those of a voltage of the utility power system to comply with islanding control standards. Since the utility power system is strong enough and excluded the operating frequency of the virtual impedance, an output current of the distributed generation power system is unaffected. But, conversely, when the utility power system is abnormal or power failure occurs in the utility power system, the strong utility power system is inexistent and there is no longer supplying the fixed operating frequency of the voltage to the load. Hence, there is only the distributed generation power system supplying power to the load so that the distributed generation power system acting as the virtual impedance is enabled. In response to the affection of the distributed generation power system, the operating frequency or amplitude of the load voltage is significantly changed. Accordingly, the islanding operation of the distributed generation power system is detected and thus the distributed generation power system is disconnected from the utility power system. In comparison with these conventional active detection methods, the detection method of the present invention can reduce a detection time of islanding phenomenon and the output current distortion of the distributed generation power system, and can increase the operating reliability.